pat_re_eval.t; test "use re 'eval'"

pat_re_eval.t; test "use re 'eval'"

In this test file, firstly reduce all "use re 'eval'"s into the smallest
scope possible, and secondly, for each pattern which still requires this
in scope, also test that pattern without it in scope, but under eval, and
check that it dies.

Note that during the course of this branch, much that formerly needed
"use re "eval'" no longer does, which is the main reason we can dispense
with it so much in this commit.